In a conventional pump of this type, the rotor is rotated by a motor which is located in a sleeve extending the exhaust end of the stator, and it thus operates at the exhaust pressure, i.e. at a primary vacuum of about 10.sup.-2 Torr, for example. The entire moving assembly is supported by bearings which likewise operate in a vacuum. Bearings may be provided at either end of the motor inside the sleeve, or else they may be provided in the suction head, with a magnetic bearing in the stator and a bearing in the sleeve beyond the motor. The exhaust end of the stator includes an exhaust pipe connected to a primary pump. At its end furthest from the stator, the sleeve is closed by an end plate.
In general, when the pump assembly (i.e. the primary pump and the turbomolecular pump) is stopped, the pump stator is returned to atmospheric pressure in order to avoid oil in the primary pump from saturating the rotor atmosphere. In order to do this, one known disposition consists in having a pipe connected to the exhaust duct, which pipe is open to the atmosphere and is fitted with an electromagnetic valve which is automatically opened when the pump assembly is stopped.
However, when the gas being pumped from the chamber connected to the suction end of the turbomolecular pump is laden with solid particles, dust, etc., these particles accumulate in the bottom of the stator around the exhaust outlet in such a manner that when the pump is stopped and returned to atmospheric pressure by the above-mentioned pipe, the dust is violently displaced and dust particles become lodged, inter alia, in the sleeve which extends the stator, and in particular in the bearings, e.g. in all bearings which are lubricated with oil or grease. As a result, the bearings wear quickly and need changing frequently.